1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image processing system, more particularly to an additional folding process mechanism which reinforces a folding line of a center-folded sheet bundle formed of a sheet of paper or the like.
2. Description of the Related Art
A sheet such as a sheet of paper printed out by an image forming apparatus such as a copying machine, a printer, or a printing machine is discharged from the image forming apparatus in one case and, in another case, made into a booklet by a center folding process in which a center part of a predetermined number of sheets being put together is stitched and folded inside.
On the other hand, in order to reinforce the folding line at a center-folded part of the booklet formed of a saddle-stitched sheet bundle, there is known an additional folding work in which a roller moving along the back of the booklet is pressed against the folding line portion.
It is known that the additional folding work is configured to press the roller against the folding line portion while moving the roller parallel to a folding line direction, the roller having a shaft direction perpendicular to the folding line of the booklet.
Having been folded in, however, the folding line portion of the booklet is bulged out by a shape restoration force generated at the time the booklet is pressurized and folded up by the roller. The bulged-out portion is thus pressurized by an edge of the roller that is in contact with a sheet end side from a stitch position, namely, the folded booklet corresponding to an upstream side of a conveyance direction, in a direction perpendicular to the folding line direction. This kind of work sometimes causes damage such as a pressurized mark left in a pressurized part.
There has been proposed a configuration, as a configuration that reduces such damage, in which the direction of the roller pressurizing the folding line portion is tilted with respect to the folding line direction (refer to Japanese Laid-Open Patent Application Publication No. 2003-341930, for example).
This configuration gets rid of the bulged-out portion and avoids leaving the pressurized mark by stretching the bulged-out portion while using a component of force that is obtained from a tilt angle and works in a direction to generate tension on the sheet of paper at the time the roller adds pressure while moving along the folding line or what is called a back of the booklet.
The configuration disclosed in Japanese Laid-Open Patent Application Publication No. 2003-341930 can get rid of the bulge at the stitch position of the sheet of paper, but a shaft line of the roller is sometimes tilted due to processing accuracy or assembling accuracy of a mechanism used to add the pressurizing force.
In this case, the shaft line is tilted not in the aforementioned folding line direction but tilted relative to the surface of the booklet, specifically in a vertical direction relative to the surface.
This kind of tilt causes an edge of the roller in a width direction to be strongly pressed against a part of the sheet, thereby sometimes leaving the pressurized mark in the pressed part. Therefore, as it stands, the damage such as the pressurized mark done to the sheet cannot be completely eliminated even when the roller is tilted with respect to the folding line direction.
Considering the problem in the aforementioned sheet processing apparatus in the related art, there is need to provide a sheet processing apparatus and an image forming system including a configuration that can completely eliminate the damage such as the pressurized mark in pressurizing the center-folded part of the sheet.